1. Technical Field
Disclosed is an electrical circuit for a photomultiplier tube (PMT) that reduces power consumption. More specifically, the invention concerns a PMT circuit comprising a low leakage switch and capacitor positioned between the resistive divider and each of the PMT electron multiplying elements (dynodes), a low power control scheme for recharging the capacitors, and a low power scheme for delivering high voltage to the PMT.
2. Related Art
Photomultiplier tubes are well known in the art having been developed well over 50 years ago. Examples are shown in U.S. Pat. No. 2,867,729 and U.S. Pat. No. 3,435,233 (herein incorporated by reference). A typical PMT includes a photocathode at one end of the tube, a focusing electrode, a series of electron multiplying elements (dynodes), and an anode. The photocathode itself comprises a photoemissive material which ejects electrons in response to photons which hit the material. An associated power supply biases the focusing electrode and dynodes to accelerate the electrons away from the photoemissive material of the photocathode and axially through the tube. As each electron hits an attracting dynode, that electron causes the dynode to eject one or more electrons. These electrons, in turn, are attracted to and hit the next high biased dynode, which ejects still more electrons. The effect therefore is to create a cascade of electrons as they move down the ladder of dynodes effect. The cascade of electrons continues through the center of the tube toward the anode. The anode collects the electrons at the other end of the tube and produces a signal indicating the amount of light or other radiation to which the photoemissive material of the photocathode has been exposed.
Conventional PMT circuits, such as, the circuit diagrams shown in U.S. Pat. No. 2,867,729 and U.S. Pat. No. 2,576,661 typically positioned the PMT anode closest to ground potential and the photocathode at the highest negative voltage. A power supply produces a single high negative voltage that is then divided down for each dynode by a series of resistors. In this prior art case, the power supply comprises a voltage source and a voltage dividing bleeder string comprising a series of resistors connected to the dynodes within the PMT. Each resistor of the bleeder string is connected to bias an adjacent accelerator stage of the PMT. This scheme, however, requires that the power supply be on continuously to supply the current drain of the resistive divider.
Another biasing method shown and described in U.S. Pat. No. 5,523,556 comprises an alternating current source and a Cockcroft-Walton (“CW”) Circuit. The CW Circuit comprises discrete elements such as diodes and capacitors, which are hard-wired in a ladder circuit. A first stage of the CW circuit multiplies the voltage of the voltage source. Successive stages of the CW circuit multiply the voltage of each preceding stage; separate stages of the ladder comprising the CW Circuit are connected to successive dynodes of the PMT. This method can draw significantly less power than circuit comprising only the resistive divider; but the leakage in the diodes is still significant enough to require a continuously energized high voltage power supply.
Still other examples are shown in the art. U.S. Pat. No. 2,594,703 and U.S. Pat. No. 2,951,941 illustrate voltage divider circuits that employ capacitors between each of the several dynode stages to prevent fluctuations in the voltage supplied by the power supply. No mention, however, is made to suggest charging the capacitors followed by powering down the high voltage source assembly in order to save electrical energy. Lastly, U.S. Pat. No. 5,880,457 illustrates a voltage divider circuit that employs the use of switches at each dynode of a PMT. The stated intent of these switches is to provide an ability to change the electron multiplication factor of the PMT and not to reduce power consumption.
What is needed, therefore, is a circuit and method for reducing power consumption to the point where it is possible to power a PMT assembly for extended periods using only a battery or some other form of stored energy.